The present invention relates to methods and apparatus for providing image data from which an image of at least a portion of a target object may be generated. In particular, embodiments of the invention relate to methods and apparatus for improving a quality of image data.
WO 2005/106531, which is incorporated herein by reference for all purposes, discloses a method and apparatus of providing image data for constructing an image of a region of a target object. Incident radiation is provided from a radiation source at the target object. An intensity of radiation scattered by the target object is detected using at least one detector. The image data is provided responsive to the detected radiation. A method for providing such image data via an iterative process using a moveable softly varying probe function such as a transmittance function or illumination function is also disclosed. The methods and techniques disclosed in WO 2005/106531 are referred to as a ptychographical iterative engine (PIE).
PIE provides for the recovery of image data relating to at least an area of a target object from a set of diffraction pattern measurements. Several diffraction patterns are recorded at a measurement plane using one or more detectors, such as a CCD or the like. A probe function, which might be a transmittance function associated with a post-target object aperture or an illumination function, must be known or estimated.
WO 2010/064051, which is incorporated herein by reference for all purposes, discloses an enhanced PIE (ePIE) method wherein it is not necessary to know or estimate the probe function. Instead a process is disclosed in which the probe function is iteratively calculated step by step with a running estimate of the probe function being utilised to determine running estimates of an object function associated with a target object.
Other methods of providing image data based on measurement of scattered radiation are also known. Embodiments of the invention may separately determine a coherent contribution and a background contribution to an estimated wavefront.
As noted above, one or more coherent diffraction patterns are recorded at the detector. However, the radiation recorded by the detector may also contain a background that does not relate to the coherent diffraction pattern. The background component may result from a variety of sources. One source of note is as a result of inelastic scattering with the object.
As incident radiation interacts with the object, a portion of the radiation interacts via inelastic scattering. At least some of the radiation having undergone inelastic scattering undergoes a change in wavelength. The change in wavelength results in the detector recording a “halo” which surrounds features formed by the elastically scattered radiation. This halo is incoherent with the elastically scattered features. Other effects from inelastic scattering may also be observed.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.